When flush_damage() is called, the output to be repainted next is
already known. Pass it along into the renderer, which can make use
of this information:
The GL renderer can get a better idea which SHM surface textures
actually have to be updated, in case a surface can be put on a plane
on one output, but not another.
A future Vulkan renderer could record texture uploads into an output
specific command buffer.
Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
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